High current commissioning of the H ion accelerating column of the ISIS injector

This paper reports on the preaccelerator of the ISIS linear accelerator injector that consists of a 665 keV medium gradient dc accelerating column which, at full specification, is required to deliver an H- beam of 650 μA mean. Although the initial operating experience with the column at low beam duty cycle was satisfactory, at high values of mean beam current the frequency of high voltage breakdown increased to an unacceptable level. An account is given of the experimental study of this phenomenon, of the mechanisms thought to be responsible and of the modifications carried out to achieve the present improved performance